Inhibition of Photosynthetic Oxygen Evolution in Thylakoids by Cyanide

Cyanide inhibition of oxygen evolution from isolated chloroplastswas accomplished by altering the thylakoid membrane properties(in addition to charge-screening) by high concentrations ofMg²⁺. The divalent cation facilitated the approach of the cyanideligand (pK = 9.4) to the water-splitting site. The additionof the divalent cation ionophore, A23187 GenBank] , in the presence ofMg²⁺ enhanced the expression of cyanide inhibition of photosystemII-mediated O₂-evolution. Furthermore, electron transport partialreactions allowed the identification of the site of cyanideaction as being between the site of Tris-inhibition of electrontransport and the site of diphenylcarbazide donation on theoxidizing side of PS II. (Received October 25, 1982; Accepted December 29, 1982)